1. Field of the Invention
The present invention relates to a fermentation process for preparing xylitol with high productivity and high yield using a novel strain of Candida tropicalis, more specifically, for preparing xylitol under optimal fermentation conditions for maximum xylitol production by optimizing the composition of medium containing xylose and the environmental conditions of culture such as pH, temperature and DO concentration.
2. Description of Prior Art
Xylitol (Xylo-pentane-1,2,3,4,5 pentol) is a naturally occurring five-carbon sugar alcohol that is present in small quantities in a wide variety of fruits, vegetables, and mushrooms. It can be produced by oxalic acid treatment of plant matters such as jute sticks and beech wood residues. However, only small amount present in natural sources, in compared with the amount of sugar cane and sugar beet, makes its quantitative extraction difficult and uneconomical.
It is also a normal metabolic intermediate of carbohydrate metabolism in mammalian. Xylitol has a number of advantageous natural properties. Xylitol has an anticariogenic property which does not cause acid formation and can promote oral health and caries prevention. Since xylitol does not need insulin to regulate its metabolism, it may be used also as a sugar substitute for the treatment of diabetes. Because its metabolism does not involve glucose-6-phosphate dehydrogenase, it is also an ideal sweetner for the person who has the deficiency of glucose-6-phosphate dehydrogenase.
On industrial scale, xylitol is produced through the chemical reduction of xylose derived from hemicellulose hydrolysates of birch wood or other xylose rich materials. As the hemicellulosic fraction of these raw materials contains polymers of other sugars, the process includes extensive purification and separation steps to remove those by-products from xylose or xylitol. The yield of xylitol is about 50.about.60%. Therefore, xylitol is an expensive product. Since the production of xylitol by the chemical method has been found to be expensive due to its difficult separation and purification steps, it is worthwhile to explore an alternative method for the effective production of xylitol using microorganisms.
To overcome such drawbacks of chemical preparation method, the biological fermentation process for preparing xylitol has been researched. The fermentation process for preparing xylitol using yeast speices from the medium containing xylose or hemicellulose hydrolysate having xylose has been disclosed. Especially, Candida blankii, Candida guilliermondii, Candida tropicalis, Candida utilis, Saccharomyces bailii, Saccharomyces rouxii, Saccharomyces uvarium and Saccharomyces pombe have been known as the microorganism producing xylitol. However, xylitol production is not quite desirable due to its slow production rate.
To improve the yield and productivity of xylitol, a high xylitol producing yeast in the present invention is isolated from the sludge of xylose manufacturing factory, and then identified as Candida tropicalis. The fermentation conditions are optimized using this strain for maximum xylitol production.